Conventionally, an excavation technique of a stratum such as the one used for boring a stratum employs, regardless of in liquid or air, turning force, impactive force, water jet or the like.
A technique employing the turning force uses an excavation bit in a ground boring machine (for example, see Patent Document 1). In this technique, the excavation bit is provided on the front edge of a rotational driveshaft and the ground is excavated by rotation and forward movement of the excavation bit. A power source for the ground excavation is rotational torque. In an excavation technique for development of petroleum and natural gas, the technique employing this rotational torque has become the mainstream.
A boring technique employing the impactive force uses, for example, a percussion drill driven on the bottom of a pit (for example, see Patent Document 2). In this technique, a drilling bit provided on the front edge of a drill string excavates by applying impact blow, or rotation and impact blow to the bit. A power source for the ground excavation is mainly impactive power, in addition to rotational torque.
A ground excavation technique employing water jet includes a shaft excavation process and a device thereof (for example, see Patent Document 3). This technique is such that a shaft is excavated by a high-pressure jet flow emitted by using water jet from a vertical nozzle provided on the end surface of a casing. A source for water jet is a high-pressure pump.
Recently, in development of underground resources in liquid, a laser has been considered for use in ground excavation. This technique is a useful approach when the liquid is highly transparent and allows a laser beam to pass through the liquid sufficiently. In laser irradiation on the ground in transparent liquid, it is possible that in an early stage, the laser beam can reach the ground to fuse and evaporate the ground. However, as fusion and evaporation of the ground progresses, the liquid begins to roil and absorb the laser beam before it reaches the ground, causing a problem that the laser beam may not reach the targeted ground. Therefore, it is considered to be difficult to bore the ground in the opaque liquid by using the laser beam.
It is known that when a laser beam is irradiated in liquid, a bubble is produced and a shock wave is generated (for example, see Non-Patent Documents 1 and 2).
It is also known that there is a close relation between a laser wavelength range and absorptance of liquid (for example, see Non-Patent Documents 3).
It is further known that when a laser beam is passed through opaque liquid, a pulsed laser beam, which has poor transmittance in liquid under ordinary circumstances, can be efficiently transmitted by using a cavitation effect (for example, see Non-Patent Document 4).
It is moreover known that irradiation of an infrared laser beam evaporates soft biological tissue to create a space and the laser beam may be transmitted efficiently through the space (for example, see Non-Patent Document 5).
Patent Document 1: Japanese Patent Laid-Open No. 2002-276276, pp., 2-4, FIG. 1
Patent Document 2: Japanese Patent Laid-Open No. 2003-184469, pp., 2-4, FIG. 2
Patent Document 3: Japanese Patent Laid-Open No. 2003-239668, pp., 2-5, FIG. 1
Non-Patent Document 1: Alfred Vogel et al., “Energy balance of optical breakdown in water”, SPIE Vol., 3254, issued on January, 1998, pp., 168-179, (an article about energy balance when a laser beam is irradiated in liquid)
Non-Patent Document 2: Alfred Vogel et al., “Shock wave energy and acoustic energy dissipation after laser-induced breakdown,” SPIE Vol., 3254, issued on January, 1998, pp., 180-189, (an article about shock wave energy and acoustic energy dissipation after laser-induced breakdown)
Non-Patent Document 3: “Wavelength range-transmission loss dependent on water content,” Latest application technology of fiber optics, issued on August, 2001, pp., 30-31, FIG. 22, (the Figure illustrates relation between a laser wavelength range and absorption of water)
Non-Patent Document 4: A. Saar, D. Gal, “Transmission of pulsed laser beams through opaque liquids by a cavitation effect,” American institute of Physics, P1556, issued in 1987, (it describes pulsed laser beam transmission through opaque liquids by a cavitation effect)
Non-Patent Document 5: Tsunenori Arai, “Evaporation mechanism of soft biological tissue by infrared laser irradiation,” T. IEE, Japan, Vol. 114-C, No. 5, 1994
An object of the present invention is to provide a novel technique for excavation using a laser beam of a submerged stratum such as a layer containing an underground resource.